1. Field of the Invention
This invention relates to a substantially single phase superconducting composition and to a method of preparing substantially single phase superconducting compositions.
2. Description of the Prior Art
The phenomena of superconductivity was first observed and reported in mercury below 4.degree. K and named by Onnes, Leiden Comm. 124C (1911). This evoked interest in discovering new materials with higher superconducting transition temperature (Tc). Initial research for superconductivity was directed at surveying elements and simple alloys to determine their superconducting properties. F. London, in J. of Chem and Physics 5 (1937), was first to speculate that supercurrents might exist in non-metal systems such as aromatic organic molecules.
During the 1950's, superconductivity research explored two principal themes: (1) development of a microscopic theory, and (2) development of empirical rules to guide the search for new superconducting materials. The first theme is exemplified by the Barden, Cooper, Schrieffer Theory of Superconductivity, Phys. Rev. 106, 162 (1957) and Phys. Rev. 108, 1175 (1957).
The second theme included development of such empirical rules as the electron per/atom, e/z ratio described by B. T. Mathias, Phys. Rev. 97 74 (1955); inverse correlations with Debye temperatures presented by J. DeLauney and R. Dolecklk, Phys. Rev. 72 141 (1947; direct correlations with specific heat noted by H. W. Lewis, Phys. Rev. 101, 93 (1956); and symmetry preferences (cubic symmetry favored over lower symmetry structures) discussed by B. T. Mathias, T. H. Geballe and V. B. Compton, Reviews of Modern Phys. 35, 1 (1963).
The decade of the 1960's extending into the 1970's saw rapid advances in superconductivity research with the potential for practical application being realized with the discovery of the Josephson effect reported in Phys. Lett. 1, 251 (1962), and the further exploration of unusual systems. In the 60's, researchers applied empirical rules and only explored the standard classes of metallic alloys and compounds.
A significant advance on the road to high Tc materials occurred in 1972 when B. T. Mathias et al. reported in Science 175, 1465 (1972) superconductivity in the composition Pb Mo.sub.6 S.sub.8. This composition is a ternary superconductor This discovery was followed in the late 70's and early 80's by the discovery of superconductivity in "heavy Fermion" systems, Steglich et al., Phys. Rev. Lett. 43, 1892, (1979) and in nearly magnetic systems, Ott et al. Phys. Rev. Lett. 50, 1595 (1983).
The possibility of superconductivity in semiconductor type materials was first predicted by Cohen, Phys. Rev. 134, A511 (1964). Shortly thereafter, Schooley et al. Phys. Rev. Lett 14, 305 (1965) reported discovery of superconductivity in SrTiO.sub.3. This was the first perovskite superconducting material.
In 1973, Johnston discovered superconductivity in LiTiO.sub.3 at temperatures as high as 13K, Mat. Res. Bull. 8, 777 (1973). This was followed by a report of superconductivity at 14K in PbBiBaO.sub.3, Sleight et al. Sol. State Comm. 17, 27 (1975). The PbBiBaO.sub.3 composition shows potential application as a sensor of electromagnetic radiation.
The discovery by Bednorz and Muller of a new class of superconducting materials in the lanthanum-barium (strontium)-copper oxide system with T.sub.c above 30K renewed interest in the field by a great number of workers. Phys. B, 64, 189 (1986). Report of Bednorz and Muller's work was followed by the report of Wu and Chu on a material in the Y-Ba-Cu-O system with a Tc above 90K, Phys. Rev. Lett. 58, 908 (1987).
Analysis of x-ray diffraction data suggested the presence of at least three phases in the Wu et al. composition. The active phase is now known to be YBa.sub.2 Cu.sub.3 O.sub.7 Cava et al., Phys. Rev. Lett. 58, 1676 (1987), Rhyne et al., Phys. Rev. B 36, 2294 (1987).
As superconducting compositions with higher Tc's are devised the well known practical applications of superconducting materials such as described in "Superconducting Machines and Services," edited by Foner and Schwartz, Nato Advanced Study Institute, Plenum, 1973, come closer to reality.